Generally, contact lenses in wide use fall into two categories: (1) hard lenses formed from materials prepared by polymerization of acrylic esters, such as polymethyl methacrylate (PMMA), and the newer rigid gas permeable (RGP) lenses formed from silicone acrylates and fluorosilicone methacrylates and (2) gel, hydrogel or soft type lenses made of polymerized hydrophilic or hydrophobic monomers, such as 2-hydroxyethyl methacrylate (HEMA). The hard acrylic type contact lenses are characterized by low water vapor diffusion constants, resistance to the effects of light, oxygen and hydrolysis and absorb only minor amounts of aqueous fluids. Because of the durability of hard contact lenses, coupled with their tendency not to absorb appreciable amounts of water, the selection of suitable disinfecting agents, cleaning agents or other lens care compounds is relatively non-critical.
However, unlike hard lenses, soft type contact lenses and certain of the newer rigid gas permeable contact lenses have a tendency to bind and concentrate significantly more fluids, environmental pollutants, water impurities, as well as antimicrobial agents and other active ingredients commonly found in lens care solutions. In most instances, the low levels of the ingredients in lens care solutions do not lead to eye tissue irritation when used properly. Nevertheless, because of the inherent binding action of protein deposits to soft lens materials, some disinfecting agents and preservatives tend to build up on lens surfaces and may become concentrated to potentially hazardous levels, such that when released could cause corneal inflammation and other eye tissue irritation.
Previous efforts to alleviate the problem of binding and concentrating disinfectants and preservatives onto contact lens surfaces, and reducing the potential for eye tissue irritation have not been totally satisfactory. For example, in spite of low toxicity levels, not all disinfectants are compatible for use with all types of contact lenses. Although they are effective antibacterial agents, their use can result in a loss of lens hydrophilic properties, cause solution instability or may even lack compatibility with certain types of hard lenses, e.g., high silicon content.
Other antibacterial agents were found to be more compatible with contact lenses and exhibit less binding on lens surfaces. In one case, it was found that chlorhexidine, a biguanide, binds to soft lens material seven times less than benzalkonium chloride, but the presence of proteinaceous oily tear-film deposits can double the amount of chlorhexidine absorbed over that of clean lenses. U.S. Pat. No. 4,354,952 discloses very dilute disinfecting and cleaning solutions containing chlorhexidine or its salts in combination with certain amphoteric and non-ionic surfactants. These solutions were found to reduce the amount of binding of chlorhexidine on hydrophilic soft contact lenses. Notwithstanding the reduction in binding achieved by this invention, the use of chlorhexidine did result in certain tradeoffs. The antimicrobial activity of the chlorhexidine may be diminished when used with certain amphoteric surfactants. Furthermore, it was reported that if not used in proper ratio, the surfactant and disinfectant will precipitate unless a non-ionic type surfactant is also employed.
U.S. Pat. No. 4,361,548 discloses a contact lens disinfectant and preservative containing dilute aqueous solutions of a polymer; namely, polydimethyldiallylammonium chloride (DMDAAC) having molecular weights ranging from about 10,000 to 1,000,000. Amounts of DMDAAC homopolymer as low as 0.00001 percent by weight may be employed when an enhancer, such as thimerosal, sorbic acid or phenylmercuric salt is used therewith. Although lens binding and concomitant eye tissue irritation with DMDAAC were reduced, it was found in some users to be above desirable clinical levels.
British patent 1,432,345 discloses contact lens disinfecting compositions containing a polymeric biguanide and a mixed phosphate buffer. The products embraced by this patent have not found acceptance by the consumer. Corneal staining is an indication of clinical acceptability and compositions as disclosed by this patent have staining values of 17% or more present, far above that which is desirable clinically.
Other efforts to reduce or eliminate soft lens binding have led to the use of anti-binding or detoxifying agents, like polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA). For the most part, however, these polymers alone were found to be ineffective in reducing lens binding and eye tissue irritation.
U.S. Pat. No. 4,758,595 to Ogunbiyi et al. disclosed that a contact-lens solution containing a polyguanide biguanide (PAPB) has enhanced efficacy when combined with a borate buffer. Such solutions are compatible with both hard and soft type lenses, and are adaptable for use with virtually any of the commonly known disinfecting techniques, including "cold" soaking under ambient temperature conditions, as well as with high temperature disinfecting methods. These disinfecting and preservative solutions are especially noteworthy for their broad spectrum of bactericidal and fungicidal activity at low concentrations coupled with very low toxicity when used with soft type contact lenses. Ogunbiyi et al. stated that biguanide polymers in the higher molecular weight ranges usually demonstrate lower toxicity levels than corresponding lower molecular weight materials.
Compositions containing PAPB and borate, or other non-phosphate buffers, have been commercialized in various products, but at levels of about 1.0 ppm or less. It is generally desirable to provide the lowest level of a bactericide possible, while maintaining the desirable level of disinfection efficacy, in order to provide a generous margin for safety and comfort.
Commercially sold contact-lens solutions containing PAPB require a regimen for the use of the product that requires a minimum of four hours soaking time. Obviously, such a soaking requirement is consistent with overnight disinfection and soaking of contact-lens, as is customarily done on a daily basis by contact-lens wearers. However, a contact-lens wearer may have occasion to remove his or her contact lenses for a shorter period of time, for example, while swimming, reading or any other activity where eyeglasses may be preferable for a particular task. Some people may wear eyeglasses for part of the day, for example, during certain work activities and may wear contact lenses for part of the day, for example, during certain social occasions. Removal of lenses at unscheduled times during the day may also occur through the need to remove environmental debris from the lens or may occur thru accidental lens removal, e.g., on occasion by blinking. In such cases, when a contact lens is installed in the eye a plurality of times during a single day, a four-hour soak may be neither practical nor convenient. In the absence of a product that provides more rapid disinfection, reinsertion of lenses that are not properly disinfected may occur. A disinfecting solution that reduces the minimum period of soaking substantially below four hours would, therefore, be desirable.
One would expect that a solution or method requiring a relatively shorter soak time for disinfecting a lens would generally require a more efficacious or stronger disinfectant than a solution or method requiring a relatively longer soak time for disinfection.
U.S. Pat. Nos. 5,411,597 and 5,411,598 to Tsao et al. disclose a contact-lens solution that is designed to disinfect a lens in a relatively short period of time. Such a solution comprises a high concentration of an alkylene glycol and an alkanol. A disadvantage of such a solution is that it is too toxic for contact with the human eye and, accordingly, must be rinsed from the contact lens before inserting the contact lens in the eye.
The stronger the bactericidal effect of a solution, however, the more likely that it may exhibit toxic effects or adversely affect comfort. In fact, many efficacious bactericides used in other contexts, such as mouthwashes, cosmetics, or the like, while being sufficiently safe for use in such products, would be too toxic for ophthalmic use, involving use in the eye. This is particular the case with soft lenses, as indicated above, because of their tendency to bind chemicals.
It would be desirable to obtain a contact-lens solution that could disinfect lenses in less time, whereby the minimum soaking time is reduced, but that would not require rinsing to remove the disinfecting solution. Some of the most popular products for disinfecting lenses are multi-purpose solutions that-can be used to clean, disinfect and wet the lenses, followed by direct "insertion" (placment on the eye) without rinsing. Obviously, the ability to use a single solution for contact lens care is an advantage. Such a solution, however, must be particularly gentle to the eye, since, as indicated above, some of the solution will be on the lens when inserted and will come into contact with the eye.
Accordingly, there is a need for improved solutions or methods for disinfecting contact lenses that can simultaneously provide both (1) a high level of antibacterial activity, such that the minimum soak period can be substantially reduced, and (2) a low order of toxicity to eye tissue, such that the solution can be used to treat contact lenses without rinsing, despite any tendency of disinfectants to bind onto lens surfaces. While challenging to develop, it would be especially desirable to obtain a method of disinfecting lenses with a multi-purpose solution for soft contact lenses, which would allow direct placement of the contact lens on an eye following brief soaking and/or rewetting with the multi-purpose solution.